Usman Muhammad pauses by the entrance to a tiny Nigerian prison cell and leans his forehead against the wall unable to speak.

The day before he had been trapped inside the stifling room, measuring roughly 6m (19.7ft) by 3m (9.8ft), with more than 50 other prisoners.

Thirty-three of his cellmates died in the cramped conditions in the prison in Minna, the capital of Nigeria’s Niger state – and four died later in hospital.

The mass deaths caused an outcry and sparked protests, including attacks on public buildings, as relatives and locals demanded answers.

Federal officials had initially blamed the deaths on a diphtheria outbreak but exactly what happened is now the subject of an investigation.

“I have never felt heat like that in my life. When we touched each other’s bodies, it felt like fire burning,” Muhammad tells the BBC once he is able to gather his thoughts.

When he was locked inside, there was standing room only amid humidity levels of more than 90%, which in some circumstances can be dangerous to human health as it prevents the natural cooling mechanism of sweat drying on the skin.

The temperature outside was above 25C and the only ventilation came from a small, high-up window.

“One of us fainted and we started knocking at the [cell] door. When [a guard] came we asked him for water. He looked at the guy that fainted and closed the door. That’s how they started dying one after the other,” Muhammad says.

Like the others who had been detained, the young man, in his 20s, is an artisanal miner, eking out a living searching for precious metals at mine sites.

The federal government has tried to crack down on this alleged illegal activity as it says it costs the country revenue, damages the environment and can be linked to wider criminal activity.

Officers from the Nigeria Security and Civil Defence Corps (NSCDC) had arrested a large group of miners near Minna last Tuesday and Wednesday.

The prisoners were held at its detention centre in the city, but the NSCDC has not shared either the names or the number of those it was holding.